Core Geometry Design - A Mental Model?

[bloomtriesskis]

Hey all - I haven't built my own skis (yet), but have been doing a lot of research to try and figure it out. One wall I'm coming up against is how to land on the right core geometry based on the material I am using and the overall feel I'm going for. I haven't found a post here that really lays it out, could someone get me pointed in the right direction? I'm mostly looking for a mental model to get me started thinking about it vs a formula that I follow every single time. I'm sure I'll experiment around as I build more and more.

Thanks!

[chrismp]

Welcome to the forum! As you already noticed, there is no real formula to this. I would highly recommend going with something simple for your first layup, that is proven to work. Like using the same triax glass top and bottom with a core type that has been used and documented on here before. For thickness profile, go with numbers that are confirmed to work well on here (paying attention to the specific layup used) or simply try to measure a ski you like and know at least the type of materials used in there. Once you have that first pair in hand, you have something to base your next iterations off of.

[bloomtriesskis]

Thanks for the reply and the encouragement to go for it. In my research around designs, I was really coming up blank on this part of the ski. I was thinking to myself that either it didn't matter that much or it was actually the secret sauce to how skis feel which is why companies talk about it a lot less when showing specs. Any sense on which of these it is?

Second, if I'm designing skis for multiple people with different heights / weights / skill level, how consistent of a feels does a single core profile give? In other words how different should the cores profiles be to account for the different skiers?

Thanks!

[chrismp]

That's definitely the secret sauce and cannot easily be described as there are so many factors playing into this.

Large manufacturers usually only offer one core profile per length and give a rather broad range of weight recommendations. If you're on the lower end of the weight recommendations a ski will feel rather stiff and vice versa. That way each person can check if a ski in the desired length would fit their weight. Have a look at what weight ranges large manufacturers recommend to get an idea what would be reasonable.

With building skis yourself you can vary core profiles a lot easier depending on the method you choose for profiling.

[bloomtriesskis]

Cool good to know.

For profiling methods, I was going to try to find and get time on a 3D router CNC - would you recommend any other methods that are either (1) more readily available or (2) more beginner friendly? I've seen a couple of videos out there of people using what looks like a mold and (what looks like) a planer to build the profile, but I'm not totally sure so I'm biased towards finding a CNC.

Thanks for the info and the help here. My plan is to spend this year messing around with how to build so that I can have something skiable by this winter. I'll be sure to keep updates posted here as this community has been super helpful so far in all of my research.

[TimW]

Very rough:
Linear taper from front/ rear binding to zero at tip / tail = evenly distributed pressure along edge
Stay thick longer = more pressure at tip/ tail
Taper down faster=more pressure near foot.

This is for a hard surface and a 'matched' stiffness.

An overall softer ski will result in the rest of the load being directly underfoot (remember: hard surface). With a given stiffness and given deflection, a constant load will be distributed. Applying more load will not change that, it will stay where you apply it.

So a constant thickness ski would result in three pressure/ contact points: tip, tail, and(if flexible enough) directly under your foot.

Note that distributed pressure is not necessarily what your looking for. Grip on ice is better with localised pressure points(think crampons).

Soft snow changes things a lot, because the snow will distribute the pressure. But you still want the ski to bend in a smooth arc for predicable behaviour (or have some camber where it does not bend)

[bloomtriesskis]

Generally, do most skis go with a linear taper? Are there advantages / disadvantages with going non - linear?

[chrismp]

I think most people start with a linear taper as its simple and usually works well, but there is no right or wrong here.

As for your question on the tools to use for profiling the core, CNC works well, but sometimes has its challenges for workholding. If you search for "profiling" on this forum, you will find a variety of different methods on how to profile cores. One user even used a bandsaw with a jig to profile cores.